CMOS imagers are increasingly being used as low cost imaging devices over charge coupled devices (CCD). A CMOS image sensor circuit includes a focal plane array of pixel cells, each one of the cells including a photo-conversion device for generating and accumulating charge in response to light incident on the pixel cell. Each pixel cell also includes devices, e.g., transistors, for transferring charge from the photo-conversion device to readout circuitry for readout.
FIGS. 1A-1B depict two adjacent conventional CMOS pixel cells 100a, 100b of an array 199. FIG. 1A is a top plan view of the pixel cells and FIG. 1B is a cross-sectional view of the pixel cells of FIG. 1A along line 1B-1B′. Pixel cells 100a, 100b are formed at a surface of a substrate 101. Substrate 101 is a p-type substrate overlying a heavily doped p-type substrate base 102. Each pixel cell 100a, 100b includes a photo-conversion device, which is depicted as a pinned photodiode 110a, 110b. The pinned photodiodes 110a, 110b respectively include a doped p-type surface layer 111a, 111b overlying a doped n-type region 112a, 112b. The n-type regions serve to accumulate charge carriers, e.g., electrons, that are generated by photons of light incident on pinned photodiodes 110a, 110b and absorbed within substrate 101.
There are sensing nodes, which are depicted as floating diffusion nodes 116a, 116b on opposite sides of a respective transfer gate 115a, 115b to pinned photodiode 110. Floating diffusion nodes 116a, 116b are doped n-type regions and receive charge transferred from the pinned photodiodes 110a, 110b by the respective transfer gates 115a, 115b.
While not shown in FIGS. 1A-1B, each pixel cell 100a, 100b also includes a respective reset transistor for resetting their floating diffusion regions 116a, 116b to a predetermined voltage before sensing a signal; and a row select transistor for outputting a signal from a source follower transistor to an output terminal in response to an address signal. CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., 256×256 CMOS Active Pixel Sensor Camera-on-a-Chip, IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., CMOS Active Pixel Image Sensors, IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). U.S. Pat. Nos. 6,177,333 and 6,204,524 also describe operation of conventional CMOS image sensors, the contents of which are incorporated by reference herein.
Adjacent pixel cells 100a, 100b, and/or other pixel cells (not shown) of array 199, can interfere with each another causing crosstalk, which results in poor image quality. Crosstalk can be either optical or electrical. Isolation techniques have been used to prevent crosstalk between pixel cells. This disclosure concerns electrical isolation techniques to prevent crosstalk. Electrical isolation is complex and depends on a number of factors including photon absorption in the substrate 101, photon wavelength, characteristics of the pinned photodiodes 110a, 110b the life-time of minority carriers, and generation and recombination centers in the substrate 101, among others.
Shallow trench isolation (STI) is one electrical isolation technique, which has been used to isolate pixels cells, as well as devices or circuitry, from one other. In general, for STI, a trench 120a, 120b is etched into the substrate 101 and filled with a dielectric to provide a physical and electrical barrier between adjacent pixels (100a, 110b), devices, or circuitry. The depth of an STI region is generally from about 2000 Angstroms (Å) to about 2500 Å.
One drawback associated with STI is crosstalk from a photon that is absorbed deep within the substrate 101 of pinned photodiodes 110a, 110b. Table 1 shows the absorption depth for photons of different wavelengths in a silicon substrate.
TABLE 1Wavelength (Nanometers)Absorption Depth (Microns)4000.194501.05002.35503.36005.06507.67008.57501680023850469006295015010004701050150011007600
Longer wavelength photons are absorbed deep within the substrate 101. Therefore, pinned photodiodes 110a, 110b must have a deeper p-n junction depth to capture the long wavelength photons. In the near-infrared and infrared regions of the spectrum, the absorption depths are high and photons travel far into the substrate 101 before being absorbed and generate charge carriers. Therefore, electrons generated by such photons must travel long distances before reaching the floating diffusion region. Accordingly, there is a greater chance that such electrons will travel to other pixel cells, causing crosstalk between adjacent pixels.
Accordingly, it is desirable to provide an improved isolation technique that prevents crosstalk from one pixel cell to another, and particularly from a pixel cell that absorbs photons having long wavelengths.